1. Field of the Invention
The present invention relates to a method for continuous tension leveling of thin aluminum strip, particularly 6000 series aluminum alloy strip.
2. Description of the Prior Art
For the leveling and flattening of metal strip, various straightening processes are used, e.g. stretch and tension straightening, tension leveling, tension-bend leveling, etc.
Generally these straightening or leveling processes are carried out continuously by means of tension stretching arrangements using a set of pull rollers and a set of brake rollers and between these sets of rollers stretch-forming forces are applied. During the stretching procedure, the strip is subjected to plastic elongation resulting in a reduction in the strip width and thickness.
It is also known to apply bending forces to the strip under tension, e.g. by bending and reverse bending over a series of small rollers positioned between the pull rollers and the brake rollers. Some prior studies have found that these multiple bendings result in an accumulation of plastic strains causing substantial work hardening. This typically leads to an increase in yield strength and a reduction in formability.
One device for stretching a continuously moving thin metal strip is described in Noxc3xa9, U.S. Pat. No. 3,867,826. A further system for continuous tension stretching of thin metal strips is described in Noxc3xa9 et al. U.S. Pat. No. 5,182,931. A general review of the leveling of aluminum strip can be found in A. L. Noxc3xa9, xe2x80x9cNew Developments in the Leveling of Aluminum Stripxe2x80x9d, Second International Aluminum Conference and Trade Exposition, Atlanta, Ga., 503-529 (1997).
These prior studies have generally found that softer aluminum alloy strips such as those formed of series AA 1000 and AA 3000 alloys can be run in a pure stretch mode, although harder AA 5000 series alloys are more successfully run in the tension leveling mode with flex rolls.
Yet another study on the action of roller levelers may be found in N. H. Polakowski, xe2x80x9cRestoration of Ductility of Cold-Worked Aluminum, Copper and Low-Carbon Steel by Mechanical Treatmentxe2x80x9d, Proceedings, Am. Soc. Testing Mats., pp 1086-1097 (1952). Here tests were conducted on the bending of aluminum rods. This researcher found that cold-worked rod material made from the softer AA 1000 and AA 3000 series aluminum alloys tended to soften when subjected to reverse bending cycles.
Thus, while there appears to be inconsistencies among the studies, in the tension leveling of thin aluminum alloy strips, the prior art has generally concluded that the softer 1000 and 3000 alloys can be run in pure stretch mode while the harder 5000 series alloys require the tension leveling mode.
According to the present invention, it has surprisingly been discovered that thin aluminum alloy strip or sheet of the AA 6000 series does not behave in the expected manner. The 6000 series aluminum alloy contains both magnesium and silicon as main alloying elements and is generally considered to be harder than the 5000 series alloy which contains only magnesium as the main alloying element. It was therefore expected that 6000 series aluminum alloy sheet would require more reverse bending action during the leveling process than the 5000 series sheet. However, it has been found that the 6000 series aluminum ally sheet or strip achieves the best combination of flatness and formability when the pure stretch mode is used or tension leveling with a low degree of reverse bending.
Thus, the present invention in its broadest aspect relates to a method for continuous tension leveling of AA 6000 series aluminum alloy sheet in which the sheet is passed through a set of pull rollers and a set of brake rollers to subject the sheet to longitudinal tension therebetween. A series of small rollers may also be located between the pull rollers and the brake rollers for subjecting the sheet under longitudinal tension to bending and reverse bending over the rollers. Best results are obtained with the AA 6000 series alloy when the system is operated in a pure stretch mode (0xc2x0 wrap angle) or at a wrap angle about the small rollers of no more than about 12xc2x0.
For tension leveling, the alloy strip typically has a thickness in the range of about 0.1 to 4.0 mm, preferably about 0.8 to 2.1 mm. The strip is subjected to a longitudinal stretch of about 0.4 to 1.0%, preferably 0.8 to 1.0%.